New architectures for integrated optics: low-loss tight bends and on-chip high-index-contrast potassium double Tungstate waveguides

This thesis concentrates on improving the performance of low-index-contrast waveguides in terms of reducing the bend losses and increasing the index contrast of waveguides by heterogeneous adhesive bonding and thinning. In the first part of this thesis, we have demonstrated that introducing a thin metallic layer underneath a tight bent polymer waveguide considerably reduces the bend losses, especially for transverse-electric polarization. For proof-of-concept demonstration, the proposed design is implemented on optical waveguides made of epoxy-based negative photoresist, SU-8. Alternatively, the same concept can be applied to various optical waveguide platforms that suffer from high bend losses due to a low refractive index contrast between core and cladding. The wavelength of interest in this study is 1.55 µm. The numerical study carried out in this work indicates that the introduction of a thin gold layer underneath the polymer channel blocks the radiation of the mode and pushes the confined mode back towards the core. The numerical studies and experimental realizations of such low-loss SU-8 based sharp bent waveguides, including fabrication and characterization, is covered in detail in the thesis. The second optical architecture is the on-chip, small-footprint high-index-contrast undoped/doped potassium double tungstate, KY(WO4)2, waveguides enabled by a novel fabrication technique. Up until now, the fabrication of rare-earth ion doped optical devices in potassium double tungstate material are based on growing a doped layer by liquid phase epitaxy on a bulk undoped KY(WO4)2 substrate. The resulting waveguide architecture has a low index contrast between the active layer and the undoped substrate, which leads to large dimensional cross-sections and a large pump power requirement to fully invert the amplifier core. In this thesis, a heterogeneous integration technique was proposed for the development of high-index-contrast waveguides amplifiers that can be potentially integrated on passive integration platforms. A novel fabrication procedures based on heterogeneous adhesive bonding and thinning is realized. Thin (~2 μm) layers of undoped and erbium doped potassium double tungstate, KY(WO4)2 (n~2), are successfully transferred onto SiO2 (n~1.44) substrates. The very first rib passive and active waveguides are fabricated on the transferred material by focused-ion-beam milling and argon plasma etching techniques.